Because the Norwalk and related caliciviruses have yet to be cultivated in any tissue culture system, along with the recent expression of 27nm virus-like particles belonging to some members of this group of agents, as well as studies of expressed viral proteins of rotavirus, the electron microscope continues to have an important role in the research program of the Epidemiology Section. It continues to be the only method (i) for direct detection of the Norwalk and related viruses and derivative 27nm expressed virus-like particles and (ii) for unraveling the antigenic relationships among these fastidious agents. The continued need for electron microscopic support was evidenced by 43 individual experiments carried out by electron microscopy since last year?s annual report.The electron microscope was used in many capacities in the past year in individual experiments as an adjunct to other studies such as examination of various calicivirus preparations for antigenic relationships by immune electron microscopy, and examination by direct or immune electron microscopy of specimens from studies aimed at expressing virus-like particles of the Snow Mountain virus.